The invention relates to switched reluctance ("SR") motors and, more particularly, to an apparatus for determining rotor position, without the use of a separate rotor shaft position sensor, for controlling when to commutate each phase of an SR motor for a desired speed at a given load. SR motors have multiple poles on both the stator and the rotor. There are windings or coils on the stator poles. Each pair of windings on diametrically opposite stator poles is connected in series to form an electrically independent phase of the SR motor. There are no windings or magnets on the rotor. However, the rotor is made up of magnetically permeable material such as, for example, a ferrous alloy.
In order to commutate an SR motor at high speed, it is first necessary to determine the position of the rotor with respect to the stator. The position of the rotor with respect to the stator establishes when the next phase should be energized. If the position of the rotor is not correctly determined, incorrect energization of the stator phases will result in inefficient operation of the motor or reduce the peak operating speed of the motor. However, conventional rotor shaft sensors for determining rotor position are bulky, unreliable and expensive.
One attempt at determining the position of the rotor with respect to the stator revolved around providing a phase current pathway to allow the phase current in the motor winding to continue to recirculate after that particular phase was energized. This current is called regen or regenerated current and the recirculation of regenerated current in the phase winding is known as free wheeling. Were it not for energy losses in the circuit, regenerated current or regen would continue to flow in the winding indefinitely.
Regen detection may be used to provide an indication of rotor pole position. When the voltage in the phase is constant, as the rotor pole moves away from the stator pole, the inductance in the stator phase winding decreases, thereby causing the regenerated current to increase in accordance with the equation: EQU V=L (dI/dT)
where V=voltage, L=inductance and (dI/dT) is the change in current with respect to time. This change in phase inductance produces a distinct bell-shaped regenerated current waveform (shown in FIG. 3).
By looking for the upturn in the regenerated current waveform at or shortly after alignment of the rotor pole with the stator phase winding, the exact position of the rotor can be determined. However, this technique generally works only for very low speed motors because the regenerated current remains in the phase winding after the rotor pole has passed the alignment position with the stator pole The existence of the regenerated current in the phase winding after alignment with the rotor pole produces braking action and torque ripple and thereby limits the practical speed range of the motor.